The present invention deals generally with measurements of the roll rate and roll angle of spinning platforms, including spinning projectiles, spin stabilized spacecraft, and other such vehicles, using the signals transmitted by the satellites of the Global Positioning System (GPS).
In the context of this application, the term “roll” is understood to mean the platform's rotation about its spin axis. The words “platform,” “vehicle,” and “projectile” are used interchangeably in this specification and are to be interpreted as inclusive, so that the mention of one also means the mention of the others. The term GPS also is to be construed broadly, and includes not only GPS but all Global Navigation Satellite Systems (GNSS) using CDMA (Code Division Multiple Access) technology.
The focus on roll results from the fact that many projectiles aimed at a distant target do not require a full attitude reference system. If the projectile is stable under all flight conditions encountered, it may not require any stability augmentation about its two cross axes, pitch and yaw. Every guided platform, however, requires a measurement of its roll angle or roll rate, since this information helps relate the guidance commands, which are executed in the platform's body coordinates, to its location in space, which is identified in geographic coordinates.
The requirement for roll information on a guided projectile differs with the projectile configuration and its guidance concept. A fully controlled projectile is one that can correct its trajectory in both the downrange and cross-range directions. To do so, it must be able to generate lift in both the upward and lateral directions. This requires knowledge of which way is up and which way, for example, is to the right. The guidance corrections are relatively insensitive to the vehicle's elevation and traverse Euler angles, but very sensitive to its roll angle. Reasonably accurate roll angle information is required. Since the command to lift the spinning projectile in a particular geographic direction is transformed into the projectile's body coordinates for execution by its control actuators, such as aerodynamic fins, this means that a fully controlled projectile—or at least that section of it that houses the control system—cannot be spinning faster than the bandwidth of its actuators.
Other methods of making downrange corrections to the platform's trajectory include adjusting the drag on the projectile. Limited corrections of cross-range may also be achieved by adjusting the projectile spin rate, which leverages the spinning projectile's natural tendency to drift in the lateral direction. This configuration requires a relatively high spin rate, typical of gun-launched projectiles. But these corrections do not require roll angle information—only roll rate is needed. So the usual requirement of the roll estimation system is either for roll angle at relatively low spin rate or for roll rate at relatively high spin rate.
Another application requiring reliable roll angle information is a spin stabilized spacecraft, which has well-controlled spin about one axis and very little motion about the other two.
Traditional methods of measuring roll rate and angle are expensive, and can be justified only for very high value platforms. An easily implemented and cost-effective solution is required for low cost projectiles, many of which spin at very high rates (for example, 300 Hz or more). Inertial rate indicators, including MEMS (MEMS=Micro Electro Mechanical Systems) gyroscopes, are relatively expensive on this scale of costs. More importantly, they require calibration prior to use, which adds to the procurement cost. Magnetometers, likewise, are expensive, besides also being susceptible to interference from local magnetic fields, such as from on-board electromagnetic actuators and other components.
For cost-effective performance, GPS, singly or in combination with other measurement techniques, continues to be investigated for determining roll and attitude of rotating platforms. GPS carrier phase and signal strength measurements are the two main techniques used for attitude determination. The preferred technique of phase difference processing typically entails fixedly attaching an array of two or more antennas at different locations on a planar surface of the platform. The separation distances between the antennas, referred to in the art as baselines, typically exceed many wavelengths of the GPS signal. The carrier phase differences between the signals received on the antennas resulting from the spatial separations (or different pointing directions) of the antennas are exploited to determine the attitude of the vehicle.
The phase differences are related to the differences in path lengths from the GPS satellite to the antennas, and several such path length differences define the platform attitude. Since, however, the prior art baselines are typically long compared to the GPS signal wavelength, accurate counts of the number of integer wavelengths in the received signal paths are required (in addition to the fractional wavelength determinations), for calculating the true path lengths (and hence phases) of the signals reaching the respective antennas.
A single antenna fixedly attached to the platform can also be used for determining roll angle or roll rate. When the platform rotates, the GPS signal received at the single antenna shows time varying characteristics, which provides information for roll determination. The power or carrier phase of the signal received at the single antenna from the GPS satellite shows a modulation over the antenna's spin cycle, whose period is a measure of the vehicle spin rate. Also, the power of the received signal is maximum at the roll angle which orients the antenna boresight nearest to the Line of Sight (LOS) to the satellite. Since the direction to the satellite is known in Earth-fixed coordinates, this determines the roll angle of the platform.
A number of these prior art techniques using GPS signals alone have been applied to satellites and space vehicles for medium accuracy attitude determination, as low-cost alternatives to the more traditional and expensive methods employing star trackers and sun- or earth-sensors. The attitude environment of these platforms is relatively benign, since space vehicles are controlled to rotate very slowly. Even spin stabilized spacecraft typically rotate at only 1 or 2 Hz. For more dynamic applications, GPS-aided inertial attitude reference systems are employed. The inertial system accurately tracks fast and rapidly changing rotations, while GPS stabilizes the long-term bias drifts characteristic of inertial instrumentation.
The present invention nominally uses only GPS signals to measure the roll rate and roll angle of a stabilized or spinning vehicle, with standard GPS receiver hardware and special purpose processing of the received GPS signals. The customary non-GPS attitude measurement sources, such as inertial instruments (gyroscopes and accelerometers) and magnetometers, are optionally provided within this approach to augment and improve upon the navigation and roll solutions obtained purely through analysis of the GPS signal data. Although applicable to a wide range of rotating platforms with varying baselines, the present invention offers the much needed, but hitherto unavailable, cost-effective solution for measuring the roll angle and roll rate of small projectiles spinning at high rates.
The present invention maintains visibility of the GPS satellites at all roll angles, using multiple antennas disposed on the platform's body about its spin axis at or near a single location along its length. For small platforms, the spacing among the antennas is necessarily small, and may be a small fraction of the GPS signal wavelength. The present invention not only computes the normal navigation solutions as to the platform's position and velocity, it also processes the received data in a unique way to produce measurements of vehicle roll angle and roll rate.